Plasma jet generator



Dec. 21, 1965 Filed Dec. 12, 1962 United States Patent 3,225,245 PLASMAJET GENERATOR Sac-hi0 Takei, Kodaira-shi, and Manabu Yarnamoto,Odawara-s'ni, Japan, assignors to Kabushiki Kaisha Hitachi Seisakusho,Tokyo, Japan, a joint-stock company of Japan Filed Dec. 12, 1962, Ser.No. 244,187 Claims priority, application Japan, Dec. 16, 1961, 36/62,089 5 Claims. (Cl. 313-231) This invention relates to techniques inspectroscopic analysis, and more particularly it relates to a new plasmajet generator so adapted as to accomplish spectroscopic analysis bymeans of the emission of light obtained by introducing an atomizedsolution sample into a high temperature plasma.

Among the devices in practical use at present as spectroscopic lightsources, there are those utilizing electrical discharge and thoseutilizing chemical flames.

In the case of the former light source, that is, by the electricaldischarge method, it is possible to create temperatures as high as 5,000to 6,000 degrees K. in a relatively easy manner, wherefore it ispossible to excite almost all elements and cause them to emit light. Onthe other hand, however, this method has the disadvantage of erosion ofthe electrodes due to bombardment of electrons or ions, this erosionbecoming particularly severe when the electrodes come into contact withsuch substances as air and water which can react chemically with theelectrodes. Since erosion of the electrodes causes instability ofdischarge and a drop in measurement precision, the introduction of thesamples in the discharge excitation method presents a great problem. Todate, however, a device fully solving this problem has not beenproposed.

In contrast to the above method, the flame photometric analysis methoddoes not require high-temperature electrodes, wherefore it is possibleto introduce samples in a relatively easy manner. For example, a commonpractice is to dissolve a sample in an acid or water to form a solution,to atomize this solution by means of an atomizer, and to introduce theatomized solution into the high-temperature region of a flame. Thisflame photometric method, however, is disadvantageous in that maximumtemperatures of only 2,000 to 3,000 degrees K. can be attained. At suchtemperatures, the kinds of elements which can be excited thereby arelimited. Although this method is relatively sensitive in the case ofsuch substances as alkali metals and alkaline earths, its application isdifficult in the case of many heavy metals.

In a recently proposed method, a plasma jet is used as a new lightsource wherein the high-temperature feature of discharge excitation andthe stable characteristic of flame photometry are combined. In thislight source, high-temperature plasma created by are discharge within aninert gas is caused to be ejected from a small hole provided in anelectrode, and the high-temperature plasma flame of several thousands toten thousand degrees Kelvin created thus is used in place of theconventional chemical flame. In the reduction of this analysis techniqueto practice, it is necessary, ordinarily, to satisfy the the followingconditions.

(1) In an arc-discharge operation, since the arc feet and particularlythe cathode are at a high temperature, it is necessary to provideprotective means for preventing erosion of the said cathode. Especiallyduring the introduction of the sample into the high-temperature plasma,this sample must be prevented from contacting the hightemperatureelectrodes.

(2) In many cases, the arc feet, especially the anode foot, are notfixed in one place but shift to various places on the electrode in anirregular manner with respect to time. Such a shifting of the arc footcauses the discharge to become unstable and lowers the reproducibilityof measurement. Accordingly, it is necessary to fix the arc foot by somemeans.

(3) When the atomized solution of the sample is introduced into thehigh-temperature plasma, the sample is vaporized and dissociated intoatoms, is then excited, and radiates. In order that this process mayproceed effectively, it is necessary that the sample penetrate as deeplyas possible into the central, high-temperature region of the plasma.

It is very difiicult to fulfill all of the above-stated requirements.For example, since the region of maximum temperature of the plasma jetis in the proximity of an electrode, and the temperature decreasestoward the extreme end of the jet, it is desirable that the sample beintroduced into the plasma as closely as possible to the electrode.However, such an introduction of the sample necessarily increases thepossibility of the sample contacting the electrode.

The term are foot appearing in this specification is explained asfollows:

An are generated in two electrodes forms an are discharge column betweenone point of one electrode, e.g., the anode, and a point on the otherelectrode, e.g., the cathode. On these two points, a movement of chargedparticles occurs between the arc discharge column and the respectiveelectrodes. Each of these two contact points between the arc dischargecolumn and the respective electrodes is called an arc foot. The contactpoint created on the anode is called the anode arc foot and the one onthe cathode the cathode arc foot.

In view of the above consideration, it is an object of the presentinvention to provide a new plasma jet generator having electrodes whichare constructionally arranged so as to fulfill in an optimum manner thevarious conditions which must be met by a plasma jet to be used as aspectroscopic light source.

The nature, principle, and details of the invention will be clearlyapparent by reference to the following detailed description of onerepresentation embodiment when taken in conjunction with theaccompanying drawing, which is an elevational view, in vertical sectionand in diagrammatic form, showing the embodiment of the generator forplasma jet spectroscopic source according to the invention.

Referring to the drawing, the anode is a water-cooled copper anode 1having formed therein an ejection nozzle 2 for the plasma jet. Theupstream part of the nozzle 2 is formed as a hollow chamber within theanode 1, and a cathode 3 in the form of a rod with a tip 4 extends fromthe bottom of the generator into the said hollow chamber so as toconfront the anode 1. During operation, an arc discharge is created withthetip 4 and a point 5 on the peripheral wall of the anode nozzle asfoot points.

A gas inlet 6 is provided to deliver an inactive gas (a gas here definedas being chemically inactive and harmless, in a practical sense, withrespect to the electrodes) into the lower part of the said hollowchamber, becoming a gas current 7 covering the cathode 3 and the innerwall of the nozzle 2, thereby protectively shielding the electrodes 1and 3. The electrode 1 is cooled by water entering its jacket through aninlet 8 and leaving through an outlet 9.

An important feature of the instant generator is a side tube 10 forintroducing samples into the plasma. For this purpose, the samplesolution is first placed in an atomizer and atomized by an inactive gas,such as argon, and the mixture of inactive gas and the sample thusformed is injected into the anode nozzle through the side tube 10. Theopening 11 of this side tube 10 is located at one part of the anodenozzle and is directed toward the high-temperature region at theinnermost part of the hightemperature plasma generated by are discharge,that is, directly at the are foot at the tip 4 on the rod-shapedelectrode 3. Accordingly, the sample is introduced in the most effectivemanner into the hightemperature plasma.

It is necessary to prevent the sample introduced in the above-describedmanner from contacting the arc feet, but in the case of the cathode footat the tip 4, there is little possibility of this contact occurring.That is, the sample flowing out from the opening 11 of the side tube 10is immediately swept away by the inactive gas current 7 and flows outtoward the nozzle 2 above, and, although it goes to a point in the closeproximity of the arc foot at the tip 4 it does not reach this are footat the tip 4. Furthermore, the sample flowing out of the opening 11 isswept away by the plasma jet toward the nozzle 2 before the sample canreach the anode foot point on the opposite side. Accordingly, there islittle possibility of the sample contacting the anode foot 5.

The fundamental condition for the proper operation in this manner of thegenerator according to this invention is that the position of the anodefoot 5 be constantly on the side directly opposite the side tube opening11. If, as a supposition, the anode foot 5 should ever shift to aposition near the side tube opening 11, the contacting of this anodefoot by the sample substance cannot be avoided. However, in thegenerator according to this invention, the anode foot 5 is pressed bythe gas flow flowing out from the opening 11 and is constantly blownagainst and fixed at the side opposite the opening 11.

In the above-described manner, the auxiliary gas flow containing thesample which is introduced through the side tube 10 fulfills thefunction of fixing the arc foot 5 and stabilizing the plasma jet as wellas the function of introducing the sample substance to a point directlyin front of the arc foot at the tip 4 which is the maximum temperatureregion of the plasma jet. This auxiliary gas flow, operating incooperation with the main gas flow 7, which shields the arc foot points4 and 5 and, at the same time, supplies the ionized gas of the plasmajet, makes possible the introduction of the sample substance into theregion of maximum temperature of the plasma flame without causing thesaid sample substance to contact either, of the arc foot points 4 and 5.Thus, it will be apparent that the generator of this invention iscapable of satisfying all of the afore-mentioned requirements.

For completely satisfactory operation of the present generator, the flowrates of the gas flows at the side tube opening 11 are extremelyimportant, and when these flow rates are suitably regulated, highlystable operation is obtained.

In order to indicate still more fully the nature of the presentinvention, the following example of specific construction and operationis set forth, it being understood that this example is presented asillustrative only, and that it is not intended to limit the scope of theinvention.

Example A plasma jet generator with an anode nozzle of 5 mm. throatdiameter, and a cathode of 7 mm. diameter was used. For the dischargegas, argon gas was supplied at a flow rate of 7 liters/min. through theinlet 6. As samples to be analyzed, dilute solutions of CaCl MgCl ZnSOand C080 and other substances were prepared. In the case of each sample,its solution thus prepared was atomized by a separate argon gas currentand this argon gas current carrying the atomized sample solution wassupplied at a flow rate of 2 liters/min. through the side tube 10.

It was found that, under these conditions, the introduction of thesample had no effect whatsoever on the stability of the operation of theplasma jet. Spectrograms taken of the light emitted by the plasmaejected from the anode nozzle 2 under the above-described conditionsclearly show atomic lines of the introduced sample metals to beanalyzed, that is, Mg, Ca, Zn, Co, and others. It was found, moreover,that the present generator produced excellent sensitivity in the case ofsuch heavy metals as cobalt and Zinc, the detection of which by flamephotometry has heretofore been especially dilficult.

A significant point which should be fully noted here is that, togetherwith the emission lines of the introduced metal sample, the spectrum ofhydrogen atoms produced by the dissociation of the water content of thesample solvent was observed, and each of these lines indicated a largebroadening due to the Stark effect. This evidence indicates that thesample had penetrated deeply to the high-temperature region at thecenter of the plasma.

In addition, the plasma jet temperature was measured by a spectroscopicmethod and was found to be approximately 7,090 degrees Kelvin at theoutlet of the anode nozzle when the arc current was amperes.Furthermore, the spectrum of the anode material did not appear in thespectrograms clearly indicating that the sample was not eroding thehigh-temperature arc foot point. The above generator was disassembledafter a long period of operation, and the electrodes were examined,whereupon it was found that neither the cathode nor the anode exhibitedany signs of erosion whatsoever.

Thus it will be seen that the present invention provides a plasma jetgenerator having a highly sensitive and stable operation.

Although this invention has been described with respect to a particularembodiment and example of application thereof, it is not to be solimited as changes and modifications can be made therein which arewithin the full intended scope of the invention, as defined. by theappended claims.

What is claimed is:

1. A plasma jet generator comprising a hollow electrode having a nozzleportion, a rod-shaped electrode disposed around the central portion ofthe said hollow electrode and pointed toward the said nozzle portion, afirst means for creating an arc discharge between the tip of the saidrod-shaped electrode and the inner wall surface of the said hollowelectrode, a second means for introducing a first inactive gas currentinto the space between the said two electrodes in the direction towardthe said nozzle portion, a third means, pointed toward the said tip ofthe said rod-shaped electrode, for introducing a second. gas current ina direction opposite to the said first inactive gas current such as tocause suppression of fluctuation of the path of the said are discharge,thereby effecting the mixture of said first with said second gas currentto the hottest point in the arc.

2. A plasma jet generator comprising, in combination, a hollow electrodesubstantially cylindrical at its bottom, tapering conically in theupward direction and ending in a nozzle; a rod-shaped electrode disposedwithin said hollow electrode having a tip which ends near the top of theconical taper in said. hollow electrode; means for creating an arcdischarge between said tip and the inner wall surface of said hollowelectrode substantially at the point where taper and nozzle meet; aninlet for an inactive gas leading into the cylindrical part of saidhollow electrode; a sample inlet leading from the top of said generatorto substantially the beginning of said nozzle and being connectedthereto; said sample and inactive gas meeting from opposite directionsabove said are discharge at the hottest point of said generator, thusavoiding contact of said sample with the arc foot points; and coolingmeans for said generator.

3. In a plasma jet generator including a hollow electrode; a rod-shapedelectrode disposed therein; means for creating an arc discharge betweenthe tip of said rodshaped electrode and the inner wall surface of saidhollow electrode; an inactive gas inlet; a sample inlet; a nozzle insaid hollow electrode; and cooling means for said generator; theimprovements which comprise said hollow electrode being substantiallycylindrical at its bottom, tapering conically in the upward direction upto contact with said nozzle; the tip of said rod-shaped electrode beingdisposed near the top of the taper of said hollow electrode; said aredischarge havin a foot point at the meeting of taper and nozzle in saidhollow electrode; said inactive gas inlet heading into said cylindricalpart; said sample inlet leading from the top of said generator tosubstantially the point at which said nozzle starts and being connectedto said. nozzle; said sample and said inactive gas meeting from oppositedirections above said are discharge thus avoiding contact of said samplewith the arc foot points.

4. In a plasma jet generator including a hollow electrode substantiallycylindrical at its bottom, tapering conically in upward direction andending in a nozzle; a rod-shaped electrode disposed in said hollowelectrode; means for creating an arc discharge between the tip of saidrod-shaped electrode and the inner wall of said hollow electrode; aninactive gas inlet leading into said cylindrical part; a sample inlet;and cooling means for said generator; the improvements which comprisesaid are discharge having a foot point substantially at the meetingpoint of nozzle and taper in said hollow electrode; said sample inletleading from the top of the generator at an angle with the generatoraxis to substantially the point where said nozzle starts and beingconnected thereto; said sample and inactive gas meeting from oppositedirections above said arc discharge at substantially the hottest pointof said generator thus avoiding contact of said sample with the arc footpoints.

5. A plasma jet generator comprising, in combination, a hollow electrodesubstantially cylindrical at its bottom, tapering conically in theupward direction and ending in a nozzle; a rod-shaped electrode disposedwithin said hollow electrode having a tip which ends near the top of theconical taper in said hollow electrode; means for creating an arcdischarge between said tip and the inner wall surface of said hollowelectrode substantially at the point where taper and nozzle meet; aninlet for an inactive gas leading into the cylindrical part of saidhollow electrode; a sample inlet leading from the top of said generatorat an angle with the generator axis to substantially the beginning ofsaid nozzle and being connected thereto; said sample and inactive gasmeeting from opposite directions above said are discharge at the hottestpoint of said generator, thus avoiding contact of said sample with thearc foot points; and cooling means for said generator.

References Cited by the Examiner UNITED STATES PATENTS 3,077,108 2/1963Gage et al 3l3231 X GEORGE N. WESTBY, Primary Examiner.

DAVID J. GALVIN, Examiner.

1. A PLASMA JET GENERATOR COMPRISING A HOLLOW ELECTRODE HAVING A NOZZLEPORTION, A ROD-SHAPED ELECTRODE DISPOSED AROUND THE CENTRAL PORTION OFTHE SAID HOLLOW ELECTRODE AND POINTED TOWARD THE SAID NOZZLE PORTION, AFIRSTT MEANS FOR CREATING AN ARC DISCHARGE BETWEEN THE TIP OF THE SAIDROD-SHAPED ELECTRODE AND THE INNER WALL SURFACE OF THE SAID HOLLOWELECTRODE, A SECOND MEANS FOR INTRODUCING A FIRST INACTIVE GAS CURRENTINTO THE SPACE BETWEEN THE SAID TWO ELECTRODES IN THE DIRECTION TOWARDTHE SAID NOZZLE PORTION, A THIRD MEANS, POINTED TOWARD THE SAID TIP OFTHE SAID ROD-SHAPED ELECTRODE, FOR INTRODUCING A SECOND GAS CURRENT IN ADIRECTION OPPOSITE TO THE SAID FIRST INACTIVE GAS CURRENT SUCH AS TOCAUSE SUPPRESSION OF FLUCTUATION OF THE PATH OF THE SAID ARC DISCHARGE,THEREBY EFFECTING THE MIXTURE OF SAID FIRST WITH SAID SECOND GAS CURRENTTO THE HOTTEST POINT IN THE ARC.